The invention concerns "sialons", i.e., homogeneous solid solutions of the Si-Al-O-N system, and it more particularly concerns a method of preparing .beta.'-sialons.
Over the past fifteen years many studies have demonstrated the advantage of silicon nitride as a refractory material, it being resistant to both high temperatures and to corrosion by many chemical agents. The principal difficulty in forming such a compound by hot pressing is that it is not compactable in the pure state but has to have a certain amount of an oxide such as MgO added to it, so as to form a vitreous phase during hot pressing. In 1972 a significant advance was made in the preparation of ceramic products with a silicon nitride base by subjecting mixtures of this nitride with alumina to hot pressing i.e., was found because it possible to form a solid solution with these two constituents over a large range of compositions (from 0 to 60 moles % Al.sub.2 O.sub.3).
In actual fact, according to recently published works, in particular thereto L. J. GAUCKLER, H. L. LUCAS and G. PETZOW in J. Amer. Ceram. Soc. 58-346 (1975) and K. H. JACK in J. Mat. Sci. 11-1135 (1976), the homogeneous solid solutions which can be formed in the Si-Al-O-N system are relatively numerous and correspond to various compositions and structures. Among these are the products obtained by reaction of silicon nitride Si.sub.3 N.sub.4 with aluminium nitride AlN and alumina Al.sub.2 O.sub.3, in equimolar proportions, i.e., Al.sub.3 NO.sub.3, constituting homogeneous crystals with the general formula Si.sub.6-Z Al.sub.Z O.sub.Z N.sub.8-Z with the same structure as .beta. silicon nitride and usually denoted by the same .beta.'-sialon. The region of existence of such a solid solution is relatively large since the value of Z can vary in practice in a continuous manner from 0 to about 4.2. Due to the good sintering quality, with or without pressure, of these .beta.' sialons, and because of the very advantageous characteristics of the ceramic materials thus obtained (refractoriness, heat resistance, resistance to corrosion, to oxidation, etc.), very many studies have understandably been devoted to them for some years. What, however, seems to limit the possibilities of application of such a material is its current cost. This cost is high, both because of the cost of the raw materials used, especially silicon nitride and aluminium nitride, and because of the expense of the sintering operation for forming the solid solution by reaction of the phases together. The sintering operation can be carried out by subjecting an intimate mixture of the powders which have an appropriate granulometry to pressing at 200 to 250 kg/cm.sup.2 at temperatures of between 1600.degree. and 1750.degree. C. for 30 to 60 minutes.
This hot pressing technique, which is both expensive and unsuitable for manufacturing parts with complex shapes, can be replaced by a technique of sintering without pressure, which is cheaper, consisting in first compacting the mixture of powders while cold, under a pressure of 7 T/cm.sup.2 (for example) and then holding the compacted product at between 1750.degree. and 1780.degree. C. for 2 hours in a nitrogen atmosphere.
To produce .beta.' sialons in a way which is cheaper than the techniques just described, authors have proposed utilizing less expensive raw materials than nitrides, by subjecting a natural or synthetic silicate of aluminium to a "nitriding" reaction.
Thus, in a recent publication (see J. Mat. Sci. 11-P. 1972 (1976)) S. WILD discloses obtaining .beta.' sialon powders by nitriding, with a mixture of ammonia and hydrogen, a finely divided powder with a large surface area of metakaolin obtained by dehydration of kaolin at 500.degree. C. The product formed after 24 hours of treatment at 1400.degree. C. is constituted by a mixture of .beta.' sialon and aluminium nitride AlN. The latter compound can be removed by treating the powder with a solution of 2.5% caustic soda NaOH for 30 minutes at 60.degree. C. This method is limited to the product of small quantities of sialon powder (about 0.2 g by experiment) because of the difficulty in obtaining satisfactory contact between the nitriding gas (ammonia) and the grains of the powder. In addition, this process of nitriding metakaolin with ammonia or NH.sub.3 /H.sub.2 mixtures allows only .beta.' sialons which are poor in Al (Z&lt;1.83) to be obtained. Since the Si/Al ratio in the starting kaolin should lead to Z=3, it is deduced that a high proportion of aluminium is found in the final product in the nitride state AlN, which necessitates purifying the sialon by the caustic soda treatment, as indicated above.
Preceding WILD's publication, U.S. Pat. No. 3,960,581 to Ivan B. CUTLER disclosed the production of sialon from clay. In fact, in the experiments described to illustrate this invention and which consist in "nitriding" with nitrogen in the presence of carbon, either rice husk (source of silica) and hydrated aluminium chloride (source of alumina), or clay expanded with polyurethane foam, or co-precipitated silicates of alumina, an "intimate dispersion" of alumina and silicon nitride is finally obtained, because the durations indicated for the nitriding stage (3 to 4 hours at 1400.degree. C.) are too short for the true solid solution of the two phases to be formed, i.e., the homogeneous phase termed sialon. Moreover, as the invention specifies, the solid solution, i.e., the sialon, can only be obtained by subjecting the compacted powder to sintering in nitrogen at between 1300.degree. C. and 1900.degree. C.
Following this patent, at the meeting of October 3rd, 1975, of the American Ceramic Society, Mr. J. G. LEE, Mr. R. CASARINI and Mr. I. B. CUTLER presented a paper on the obtaining of sialon from clay; according to these authors, the nitriding of clay in the presence of carbon is carried out in a nitrogen atmosphere at 1450.degree. C. with iron as catalyst. The powder obtained, pressed and sintered in a nitrogen atmosphere (1 hour at 1550.degree.-1650.degree. C.) produces compact products whose density is a function of the sintering temperature (3.01 for 1550.degree. C.; 3.03 for 1600.degree. C.; 3.14 for 1650.degree. C.). Under metallographic analysis, these products, which are difficult to polish, show a dispersion of ferrosilicon in relatively significant quantity in the solid solution studied. It should be observed that in these experiments, in which, from measurement of the loss of weight, the nitriding was total at 1450.degree. C. in less than one hour, it is probable that at the end of so short a time the sialon solid solution is not more developed than in those described in the CUTLER patent and that "an intimate dispersion" of alumina and silicon nitride is again involved.
The current state of the technique for making sialons from kaolin or kaolinitic clays presents some diadvantages.
1. According to the publications mentioned above, it appears that none of the proposed techniques allows the sialon solid solution to be obtained in the state of a pure phase. The heterogeneous powders obtained according to the WILD method contain a considerable amount of aluminium nitride in addition to the solid solution which is relatively poor in aluminium.
The operations described in the works of CUTLER and al. produce dispersions of silicon nitride and alumina which are not sialons and which must be subsequently sintered at a higher temperature to produce the sialon phase. Furthermore, these sintered products contain a ferrosilicon phase when the nitriding reaction has been catalyzed by iron.
2. The nitriding operation carried out on powdery products transfers with great difficulty to an industrial scale. It can certainly be carried out on a thin layer of powder so that all the grains can be in contact with the gaseous phase constituting the nitriding agent but becomes impossible with a large mass of powder, only a small surface portion of which is in contact with the gaseous phase. The solution of using a fluidized bed which allows sufficient mixing of the whole of the powder with the nitriding gas seems an especially difficult application to the man of the art, due to the temperature at which nitriding is carried out.
The object of the present invention is to provide a new method of making .beta.' sialons presenting the double advantage, over the methods previously described, of using only very cheap raw materials and simple and inexpensive operating conditions, and also of being capable of being carried out with quantities of material which can be as large as required, i.e., it transfers very well to industrial production scale.